1. Field of the Invention
The present invention relates to a document reading apparatus for reading an image on a document in which the document is illuminated with light from a fluorescent lamp and the light reflected from the document is converted into an electrical signal by a solid-state imaging device such as a CCD (charge coupled device) sensor.
2. Description of the Related Art
In recent years, a smaller and less expensive document reading apparatus with higher quality is required.
FIG. 12 shows a configuration of a conventional document reading apparatus. The document reading apparatus which is disclosed in Japanese Laid-Open Patent Publication No. 62-195976 includes an object 53 placed on a document plate 52 for determining a reference gray scale of an image.
When a CPU 60 receives a specific command, the CPU 60 supplies a light-up signal to a fluorescent lamp lighting circuit 63 so as to light up a fluorescent lamp 51. The fluorescent lamp 51 then illuminates the object 53. Light reflected from the object 53 is converged on an image sensor 56 by a Selfac lens 55. The image sensor 56 converts the received light into an electrical signal. The output of the image sensor 56 is compared with a predetermined reference value by a comparator 58. The reference value is supplied to the comparator 58 by a reference source 59.
When the output of the image sensor 56 is smaller than the reference value, the comparator 58 outputs a HIGH signal to the CPU 60. On receipt of the HIGH signal, the CPU 60 controls a heater source 62 so as to switch on a heater 61 and thus raise the temperature of the tube wall of the fluorescent lamp 51. Thereafter, when the output of the image sensor 56 becomes greater than the reference value, the comparator 58 outputs a LOW signal to the CPU 60. On receipt of the LOW signal, the CPU 60 controls the heater source 62 so as to switch off the heater 61, and controls a motor driving circuit 64 so as to drive a motor 57. When the motor 57 starts driving, the read operation of a document 54 that is placed on the document plate 52 is started.
Alternatively, when the output of the image sensor 56 is smaller than the reference value, the CPU 60 may control the heater source 62 so that the heater 61 be kept switched on for a predetermined period. In this case, the read operation of the document 54 is started after the lapse of the predetermined period.
When the output from the image sensor 56 is greater than the reference value, the comparator 58 outputs a LOW signal to the CPU 60. On receipt of the LOW signal, the CPU 60 allows the motor 57 to start driving without switching on the heater 61, so as to start the read operation of the document 54.
As described above, the conventional document reading apparatus determines whether or not the read operation of the document should be started by comparing the output of the image sensor with a predetermined reference value. According to such a conventional document reading apparatus, the read operation of the document will not be started when the output of the image sensor does not reach the reference value due to a decrease in the amount of light emitted by the fluorescent lamp caused by changes over the years. To avoid this trouble, the reference value needs to be set to a lower level so as to compensate for this change over the years. In the latter case, however, the conventional document reading apparatus will start the read operation of the document even when the temperature of the tube wall of the fluorescent lamp is lower than the level required. As a result, the amount of light from the fluorescent lamp during the read operation of the document varies. This deteriorates the quality of the resultant image.
The conventional document reading apparatus has another disadvantage of being influenced by environmental changes. Especially, when the conventional document reading apparatus is placed under a low temperature condition, the temperature of the tube wall of the fluorescent lamp will not rise sufficiently even when the fluorescent lamp is heated by the heater. In this case, since the output of the image sensor will not reach the reference value, the conventional document reading apparatus cannot start the read operation of the document.